The main properties desired for a material intended to be used as active cathode material in a lithium battery are a high specific energy (which is the product of the capacity multiplied by the mean potential) and a long lifetime in cycling. The material Li1+xV3O8 (where (0.1≦x≦0.25) meets these criteria and many methods of preparation are described in the prior art.
It is known in particular to prepare Li1+xV3O8 from Li2CO3 and V2O5 in powder form.
Hammou, et al. [Electrochim. Acta, 13 (1988) 1719] describes a method for preparing Li1+αV3O8 in which the reactants react in air for 6 h at 590° C. However, this temperature is very close to the melting point and results in the powder sintering, requiring it to be milled before it can be used to prepare a composite electrode.
U.S. Pat. No. 6,136,476 describes a method in which the reactants are heated to a temperature below the melting point, preferably between 350° C. and 550°, after the reactants have been mixed using various means for the purpose of reducing the particle size and of making the particle size distribution uniform.
Chaloner-Gill, et al. [J. Electrochem. Soc., 147(10), 3575-3578, (2000)] describes a method comprising a succession of steps, namely: milling of the reactant mixture; heating in air at 585° C. for 16 h; cooling and remilling; second heating in air at 585° C. for 16 h; reaction with Li2S.
U.S. Pat. No. 5,520,903 describes a method consisting in milling the reactants for the purpose of mixing them and for reducing the particle size, in compressing the mixture in order to form a compact powder, and then in heating the compressed mixture at a temperature between 580 and 585° C. In this case, the product obtained is an agglomerate of single-crystal particles bonded together, which has to be milled before it can be used as electrode material. Admittedly, the milling gives free particles, but it also results in the loss of the rod-shaped single-crystal morphology, and crushed poly-crystalline particles are obtained.
FR-2 831 715 describes a method consisting in powder blending the precursors in stoichiometric proportions under conditions which give a density of less than 1.5 and particle sizes within precise ranges, in heating at a temperature between 565° C. and 585° C., which is maintained for 30 min to 10 h, and then in deagglomerating the powder obtained.
In general, the aforementioned methods for preparing Li1+xV3O8 are lengthy and, because of the high-temperature temperature heat treatment, the particles are relatively coarse, this being unfavorable to their use as positive electrode material for lithium batteries.
Other methods, employing gelled precursors, have also been studied.
G. Pistoia, et al., [J. Electrochem. Soc., 137, 2365, (1990)] teaches a method of preparing a gelled precursor of Li1+xV3O8 (where (0.1≦x≦0.25) by dissolving α-V2O5 in an aqueous LiOH solution and by heating to 50° C. in a nitrogen atmosphere. The gel forms after several tens of hours.
Jinggang Xie, et al., [Mat. Letters, 57, 2682, (2003)] teaches a method of preparing an LiV3O8 gel by adding an LiOH.H2O powder to a V2O5 gel prepared beforehand by polycondensation of vanadic acid. The acid is itself obtained by passing a sodium metavanadate (NaVO3) solution through an H+/Na+ ion exchange column. The entire process comprises a large number of steps, which are carried out over several tens of hours, with tedious and expensive use of ion exchange resin.
The various methods of preparing lithium vanadium oxide from gelled precursors comprise many steps, some of which are tedious. Furthermore, the precursors used are often expensive and their use is tricky, especially because of their toxicity.